flashing 3 phase generator 6

[holo]

I'm new to the forum.
I have a Kohler 15RMY62 208V 3 phase 15 KW generator that is producing 30 VAC (rms) at 1800 rpm.

The diodes and brushes have been replaced.

I disconnected the field windings from each other and they are 0.9 ohms each. The wiring has no visible signs of burning or discoloration.

I flashed the 4 pole rotor with a 12VDC battery thru the brushes when it was not running. No change.

Any help or advice would be most appreciated!

 

[itsmoked]

Some generators need a load before they put out more than a few volts. Have you hooked the ol'waffle iron up and tried it?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[holo]

Thanks itsmoked!
I haven't tried hooking up a load.
Waffle iron? OK, I'll try it!
Thanks!

 

[holo]

I connected a 13 ohm nichrome resistor across 1 leg to neutral. Same thing as before, 30 Vrms, 64.4 Hz. Scoped out as a good sine wave.
Could this be a voltage regulator problem? Does the rotor need to be flashed with higher voltage?

Again, any help/advice is very much appreciated!

 

[itsmoked]

Shucks I had a gen that put out about 25Vac until you dropped a load on it then 'bang' 120Vac.

Generally if you are getting 30V there is more than enough energy available to allow the generator to 'build' up to normal.

Yes it could absolutely be the VR. Generally that is where the problem lies.

First though you said 'rings' these are for the field then?

If that is so you may be able to provide some 'manual' field current and see the output rise. If you can and you see it rise dramatically never mind to 208V then you can probably assume the field is intact and the 'mechanical' generator is okay. At this point you would want to focus on the regulator. Again the regulator's job is to look at the output voltage and adjust the field current accordingly. If you could put an ammeter of some sort in the field circuit you could see if the VR was functioning. Make sure your ammeter is good for several amps. (DC)

Others will come along who sleep and eat generators. They will have some sage advice.



Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[holo]

Are you refering to a capacitor/inductor resonation? Can't find a cap anywhere except on a VR board in the unit.
"If that is so you may be able to provide some 'manual' field current and see the output rise."
I'm not sure exactly what you mean. 12 volt battery flash to the field windings and watch the ammeter? If it works, the voltage would go high in several milliseconds. I'm allergic to sulfuric acid, so I'll fuse the battery.
Thanks

 

[itsmoked]

[red]"Are you referring to a capacitor/inductor resonation?"[/red]

You totally lost me here.. Capacitor?

I am talking about providing field current with something. Disconnect the VR and just feed current to the field. A battery might work but you might need more than 12V.

I'm also talking about finding your existing system's field control wires to/from rings and inserting an ammeter in one. You should see some amps being provided by the VR.

Of course before this stuff with all power removed you should be able to confirm the field wining is not open. (Presuming you've already done this!)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

To avoid repeating a bunch of issues, please read this older thread: thread659-117376

Being that yours does have brushes we can assume that you have a synchronous generator, which makes sense since you are expecting to run it in island mode. OK then, did you realize that a synchronous generator must have power applied to the exciter at all times? Flashing is just to set up an initial field on a brand new generator or one that has been out of service for a long time where the residual magnetism in the rotor is insufficient to provide enough power to the exciter system, and is usually done off of the same battery used to start the engine. In fact, the initial flashing on most small portables can be done with very little voltage, I have done it with a drill motor as a generator in a pinch (I won't go into details unless asked). After that the exciter circuit needs to take over on its own.

The 30V output you are getting is likely the result of some small amount of residual magnetism in the rotor. The fact that it can do that means you should have enough to get the exciter going. Is your exciter properly connected? You mentioned having replaced the diodes, are you sure all the wires went back onto the same terminals? Now that I think of it, why did you have to replace the diodes anyway? If they shorted, chances are some other part of the circuit was damaged as well. Does it have an output? Does it have a Field Circuit Breaker that may be open?

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[powerwagon75]

holo,

You need to answer jraef's questions above to yourself in your troubleshooting process. Also, not being real familiar with Kohler model #'s, I don't know how old it is--how advanced of a voltage regulating system it has.

To do a conclusive flash while it is running, identify F1 an F2 wires going to the excitor. This is where you want to flash the DC in. If running a solid state regulator, you want to disconnect the leads from the regulator before doing this, or put a diode on the B+ line before flashing to avoid sparking surprises.

If you've confirmed diode installation/brushes/wiring to them is good, and flashing gives you a predicted result, investigate the regulation circuit it has. Check if sensing/power voltages are rising/falling when being flashed.

Any number of things it could be on these old small units. Just need to identify the key components, look for expected results from each, and lots of ohming to ensure all is connected from point to point.

 

[waross]

Hi folks;
First, if you try to flash the generator with the starting battery before you disconnect the voltage regulator, you will probably join a long list of people with blown voltage regulators.
But, your generator should boot up with 30 volts. It's when we are reading between zero and 3 or 4 volts that we consider flashing.
If you disconnect the voltage regulator you may connect the starting battery to the field.
Usually it takes 12 to 24 volts on the field to get full voltage at no load. It often takes twice as much voltage on the field to give full voltage with full load.
by the way, I have succsesfully flashed a 35 kw set with a 1.5 volt "D" cell in series with the voltage regulator. It doesn't take much. (The voltage regulator probably helped a little.)
It sounds as if you have regulator problems.
respectfully

 

[edison123]

If you get 30 V residual voltage, no need to flash the field. Either your field circuit is open (you mentioned "I disconnected the field windings from each other" - did you connect them back ? or your leads from the excitation circuit to the brushes is broken) or your excitation system or voltage regulator has failed.



* If a little knowledge is dangerous, then I am the safest person in the world *

 

[powerwagon75]

The field flashing procedure isn't for the sole purpose of restoring residual. It can be used as a troubleshooting method of testing the integrity of generating side of the machine. No point in throwing new voltage regulators at it if something else is dragging it down.

 

[JIMGEN]

I would think that if you only have .9 ohm across the fields you have a problem with the field windings. .9 ohms is far too low for the Kohler. Check further for shorted windings. 6v on the fields ( with the avr out of the circuit) should bring the output to well over 100volts. But ohm's law indicates that if you apply 30 volts on a .9 0hm field you would be drawing 33 amps, far too much. You have a problem with the fields or the leads to the fields.

 

[waross]

I just reread the post and was going to suggest that the field resistance was too low, but JIMGEN beat me to it.
Good call JIMGEN.
respectfully

 

[edison123]

OP said all 4 pole coils measure 0.9 ohms each. That gives a total of 3.6 ohms with 8.33 amps field current.

Given the equal resistances of the 4 pole coils, I would rule out any turn shorts.

The turn short can also be confirmed by doing ac pole drop test. (Apply 110 V AC across field winding terminals and measure the voltage drop across each pole coil. Equal voltage drops will rules out turn shorts)

* If a little knowledge is dangerous, then I am the safest person in the world *

 

[waross]

Good point edison123;
A very common but by no means universal specification for voltage regulators for small generators is Minimum 16 ohms field resistance.
If the field is good, it is possible that the voltage regulator has been replaced with one which requires a minimum of 16 ohms.
respectfully

 

[holo]

I purchased a new AVR and voltage regulator. No change in the output voltage, even with the waffle iron.
The rotor checked out at 27 ohms. My book for single phase unit shows the reading should be between 2.5 and 4.5 ohms. This sounds too low. I'm guessing it should be the same for three phase because both operate on DC voltage to the exciter rings.
With the electronics disconnected and the rotor hooked to a 12 volt auto battery, I get 46 volts out. Can't get the ammeter to work. There does not appear to be a short in the rotor.
Your help is greatly appreciated!

 

[edison123]

High rotor resistance (27 ohms as against 3.6 ohms) indicates some high resistance connection either in between poles and or in the rotor leads.

* If a little knowledge is dangerous, then I am the safest person in the world *

 

[powerjunx]

ed,

"The turn short can also be confirmed by doing ac pole drop test. (Apply 110 V AC across field winding terminals and measure the voltage drop across each pole coil. Equal voltage drops will rules out turn shorts)"
Voltage drop across each pole should not necessarily equal. It should fall within tolerance value.

holo,
Better send your generator for thorough check up if you desperately need it.

 

[waross]

Hello;
I did some googling on your model number. I found some of your other queries and got a little new information.
Apparently, your generator has a static exciter. This is not common on a set this size. The only other one I have seen was an old Caterpillar.
A static exciter may have the voltage regulator built in, or it may be basically a power amplifier controlled by a standard voltage regulator.
Voltage regulators for that size set are pretty generic and/or interchangeable.
HOWEVER, a standard, off the shelf AVR (automatic voltage regulator) will not have the current capacity to properly energise your field directly.
From the field resistance values you have supplied, and the voltages you found when testing, I strongly suspect that your problem lies in the static exciter.
You will need 31 volts in to get 120 volts out at no load. possibly twice that at full load. You will need close to 20 amps.
The 27 ohms is a worry. You should measure the rotor resistance at the slip rings and NOT through the brushes. There will be a non linear voltage drop across the brushes that will cause serious errors if you try to measure field resistance through the brushes with a multi-meter.
One other thing to check is the connections of the output or power leads. This is a long shot but it is something that should be verified.
For a DC supply you may use batteries. 6 volts, 12 volts, 24 volts, etc. The output voltage should be proportional to the input voltage.
You may also use a variable AC source and a bridge rectifier. The relatively large inductance of the field winding will act as a quite efficient filter.
respectfully